Pulp mills are usually operated to bleach the pulp to the highest possible brightness. This may allow the mill to obtain a higher price on the market or reduce costs in the papermaking process by reducing the amount of expensive additives used when making the paper, such as optical brightening agents. In practice, however, it can be difficult to consistently maintain very high brightness from the bleach plant.
Problems with obtaining and maintaining high brightness can be a result of the chemistry used in typical bleaching operations, as well as limitations resulting from design or equipment limitations. In that regard, it is common for ClO2 bleaching to stall out in later bleaching stages, e.g., the final D2 bleaching stage, where brightness no longer increases and can even decrease as the pulp is retained longer in the bleaching stage. This requires the pH to be closely controlled in order for the the brightness to be maximized for the ClO2 bleaching, which can be difficult due to very long dead time and process variation.
Also, it is common for older pulp mills to run at higher production rates than the rates they were originally designed for. In such a case, retention time in the bleaching process is lower than optimal, resulting in high residual ClO2 and relatively low brightness.
Accordingly, there exists a need to increase brightness of the pulp, while avoiding the problems discussed above.